My rosy won't eat!!! (long)

If someone could please give me some advice on the situation, I would really appreciate it. I received an el rosario rosy about 3-4 weeks ago. It is about 8 inches long and an inch and a half around. The people said that it was eating tiny pinkies so I went to the store and bought one the day after I got the snake. It didn't eat it so I put it back in the cage thinking maybe it was like snake "jet lag". Just last week I tried to feed it again and it wouldn't eat again. It looks at the mice like its' interested but it won't eat them. Some people have told me to put a guppy in its water dish or to feed it crickets. The temperatures in the cage are 65-70 on the cool side and 80-85 on the hot side. The snake is active and crawls arould a lot, so whats the problem???